Thin-section and freeze-fracture studies of crayfish stretch receptor synapses including the reciprocal inhibitory synapse

Peripheral synaptic contacts at mechanoreceptors in arachnids and crustaceans: morphological and immunocytochemical characteristics

Two types of sensory organs in crustaceans and arachnids, the various mechanoreceptors of spiders and the crustacean muscle receptor organs (MRO), receive extensive efferent synaptic innervation in the periphery. Although the two sensory systems are quite different-the MRO is a muscle stretch receptor while most spider mechanoreceptors are cuticular sensilla-this innervation exhibits marked similarities. Detailed ultrastructural investigations of the synaptic contacts along the mechanosensitive neurons of a spider slit sense organ reveal four important features, all having remarkable resemblances to the synaptic innervation at the MRO: (1) The mechanosensory neurons are accompanied by several fine fibers of central origin, which are presynaptic upon the mechanoreceptors. Efferent control of sensory function has only recently been confirmed electrophysiologically for the peripheral innervation of spider slit sensilla. (2) Different microcircuit configuration types, identified on the basis of the structural organization of their synapses. (3) Synaptic contacts, not only upon the sensory neurons but also between the efferent fibers themselves. (4) Two identified neurotransmitter candidates, GABA and glutamate. Physiological evidence for GABAergic and glutamatergic transmission is incomplete at spider sensilla. Given that the sensory neurons are quite different in their location and origin, these parallels are most likely convergent. Although their significance is only partially understood, mostly from work on the MRO, the close similarities seem to reflect functional constraints on the organization of efferent pathways in the brain and in the periphery.

Organization of efferent peripheral synapses at mechanosensory neurons in spiders

The mechanosensory neurons of arachnids receive diverse synaptic inputs in the periphery. The function of most of these synapses, however, is unknown. We have carried out detailed electron microscopic investigations of the peripheral synapses at sensory neurons in the compound slit sense organ VS-3 of the spider Cupiennius salei. Based on the localization of discrete presynaptic vesicle populations, it is possible to discriminate at least four different synapse types, containing either: (1) small round, electron-lucent vesicles 32 nm in diameter; (2) large round, clear 42-nm vesicles; (3) a mixture of small and large clear, round vesicles, similar in size to those in Type 1 and Type 2 synapses, respectively, and granular and dense-core vesicles; or (4) clear, round 37- to 65-nm vesicles. Combined immunocytochemical labeling at the light and the electron microscopic level suggests that gamma-aminobutyric acid (GABA) is the transmitter in many of the 32-nm vesicle synapses, and glutamate in many of the 42-nm ones. Based on vesicle type and particular synaptic configuration, various forms of presumed efferent synaptic contacts are distinguishable with the sensory neurons, the surrounding glia, and between the putative efferent fibers themselves. These include simple unidirectional synapses, reciprocal synapses, serial synapses, and convergent as well as divergent dyads. These various synaptic microcircuits are suited to serve a variety of functions. Among these are direct postsynaptic inhibition or excitation of the mechanosensory neurons, and disinhibition or sensitization via presynaptic inhibition or excitation. The observed synaptic configurations are compared with those at the crustacean muscle receptor organ. They reveal a remarkable complexity of synaptic microcircuits at spider sensilla and suggest manifold possibilities for subtle, efferent control of sensory activity.

Presynaptic inhibition and antidromic discharges in crayfish primary afferents

The mechanisms of presynaptic inhibition have been studied in sensory afferents of a stretch receptor in an in vitro preparation of the crayfish. Axon terminals of these sensory afferents display primary afferent depolarisations (PADs) mediated by the activation of GABA receptors that open chloride channels. Intracellular labeling of sensory axons by Lucifer yellow combined with GABA immunohistochemistry revealed the presence of close appositions between GABA-immunoreactive boutons and sensory axons close to their first branching point within the ganglion. Electrophysiological studies showed that GABA inputs mediating PADs appear to occur around the first axonal branching point, which corresponds to the area of transition between active and passive propagation of spikes. Moreover, this study demonstrated that whilst shunting appeared to be the sole mechanism involved during small amplitude PADs, sodium channel inactivation occurred with larger amplitude PADs. However, when the largest PADs (>25 mV) are produced, the threshold for spike generation is reached and antidromic action potentials are elicited. The mechanisms involved in the initiation of antidromic discharges were analyzed by combining electrophysiological and simulation studies. Three mechanisms act together to ensure that PAD-mediated spikes are not conveyed distally: 1) the lack of active propagation in distal regions of the sensory axons; 2) the inactivation of the sodium channels around the site where PADs are produced; and 3) a massive shunting through the opening of chloride channels associated with the activation of GABA receptors. The centrally generated spikes are, however, conveyed antidromically in the sensory nerve up to the proprioceptive organ, where they inhibit the activity of the sensory neurons for several hundreds of milliseconds.

Shunting versus inactivation: analysis of presynaptic inhibitory mechanisms in primary afferents of the crayfish

Primary afferent depolarizations (PADs) are associated with presynaptic inhibition in both vertebrates and invertebrates. In the present study, we have used both anatomical and electrophysiological techniques to analyze the relative importance of shunting mechanisms versus sodium channel inactivation in mediating the decrease of action potential amplitude, and thereby presynaptic inhibition. Experiments were performed in sensory afferents of a stretch receptor in an in vitro preparation of the crayfish. Lucifer yellow intracellular labeling of sensory axons combined with GABA immunohistochemistry revealed close appositions between GABA-immunoreactive (ir) fibers and sensory axons. Most contacts were located on the main axon at the entry zone of the ganglion, close to the first branching point within the ganglion. By comparison, the output synapses of sensory afferents to target neurons were located on distal branches. The location of synaptic inputs mediating spontaneous PADs was also determined electrophysiologically by making dual intracellular recordings from single sensory axons. Inputs generating PADs appear to occur around the first axonal branching point, in agreement with the anatomical data. In this region, small PADs (3-15 mV) produced a marked reduction of action potential amplitude, whereas depolarization of the membrane potential by current injection up to 15 mV had no effect. These results suggest that the decrease of the amplitude of action potentials by single PADs results from a shunting mechanism but does not seem to involve inactivation of sodium channels. Our results also suggest that GABAergic presynaptic inhibition may act as a global control mechanism to block transmission through certain reflex pathways.

Peripheral synapses at identified mechanosensory neurons in spiders: three-dimensional reconstruction and GABA immunocytochemistry

The mechanosensory organs of arachnids receive diverse peripheral inputs. Little is known about the origin, distribution, and function of these chemical synapses, which we examined in lyriform slit sense organ VS-3 of the spider Cupiennius salei. The cuticular slits of this organ are each associated with two large bipolar mechanosensory neurons with different adaptation rates. With intracellular recording, we have now been able to correlate directly the staining intensity of a neuron for acetylcholinesterase with its adaptation rate, thus allowing us simply to stain a neuron to identify its functional type. All rapidly adapting neurons stain more heavily than slowly adapting neurons. Immunostaining of whole-mount preparations reveals GABA-like immunoreactive fibers forming numerous varicosities at the surface of all sensory neurons in VS-3; peripheral GABA-like immunoreactive somata are lacking. Sectioning the leg nerve procures rapid degeneration of most fiber profiles, confirming that the fibers are efferent. Punctate synapsin-like immunoreactivity colocalizes to these varicosities, although some synapsin-like immunoreactive puncta are GABA-immunonegative. Fibers with similar immunoreactivities are also associated with trichobothria, tactile hairs, internal joint receptors, i.e. other types of spider mechanosensory organs. In organ VS-3, immunoreactivity is most dense across the initial axon segment. The exact distribution of peripheral synapses was reconstructed from a 10-microm-long electron micrograph series of the dendritic, somatic, and initial axon regions of acetylcholinesterase-stained VS-3 neurons. These reveal a pattern similar to that of the synapsin-like immunoreactivity. Two different types of synapse were distinguished on the basis of their presynaptic vesicle populations. Many peripheral synapses thus appear to derive from efferent GABA-like immunoreactive fibers and probably provide centrifugal inhibitory control of primary mechanosensory activities.

Inhibitory axoaxonal and neuromuscular synapses in the crayfish opener muscle: membrane definition and ultrastructure

The specific inhibitory motoneuron to the crayfish (Procambarus clarkii) opener muscle provides neuromuscular synapses to the muscle fibers and axoaxonal synapses to the excitatory motor nerve terminals. Freeze fracture of the membrane in both types of synapses show that the presynaptic active zone consists of clusters of large particles (putative calcium channels), which are often encircled by large depressions representing fused synaptic vesicles on the internal leaflet or P face of the presynaptic membrane. Corresponding pits and protrusions mark the external leaflet or E face of the presynaptic membrane. The postsynaptic receptor-bearing surface, characterized for neuromuscular synapses only, consists of rows of particles on both leaflets of the muscle membrane. The organization differs from that seen at excitatory synapses where particles occur only on the E-face leaflet. Serial thin sections of nerve terminals reveal that neuromuscular synapses are significantly larger in proximal fibers than in their central counterparts and support a greater number of presynaptic dense bars (active zones). Axoaxonal synapses also show regional differences; almost three times as many occur in the proximal region compared with the central region. Most synapses possess a single dense bar. The majority of synapses formed by the inhibitory axon are neuromuscular; a minority are axoaxonal. The latter occur in various locations along the excitatory nerve terminals as well as on branches of the axon itself. This preterminal or "off-shore" location could act to cut off entire populations of excitatory synapses or reduce the amplitude of the preterminal action potential.

Inhibitory innervation of a lobster muscle

Inhibitory neuromuscular synapses formed by the common inhibitor (CI) neuron on the distal accessory flexor muscle (DAFM) in the lobster, Homarus americanus, were studied with electrophysiological and electron-microscopic (thin-section and freeze-fracture) techniques. Postsynaptic inhibition as indicated by inhibitory junctional potentials was several-fold stronger on distal compared to proximal muscle fibers. This difference correlated with the results of serial thin-section studies, which showed more inhibitory synapses on distal fibers than on their proximal counterparts. Effects of postsynaptic inhibition on excitatory junctional potentials via current shunting had a morphological correlate in the spatial relationship between inhibitory and excitatory synapses on the distal fibers. Inhibitory synapses were larger than their excitatory counterparts and had fewer glial processes. In freeze-fracture views, inhibitory synapses did not appear as raised plateaus in the P-face as do excitatory synapses, and their active zones were more widely scattered. The intramembrane particles in the inhibitory postsynaptic membrane - representing neurotransmitter receptors - are arranged in parallel rows in the sarcolemmal P-face and have complementary furrows in the sarcolemmal E-face. Altogether, our findings help to describe a population of inhibitory neuromuscular synapses formed by the CI neuron in lobster muscle.

New types of synaptic connections in crayfish stretch receptor organs: an electron microscopic study

The synaptic input to crayfish (Orconectes limosus) stretch receptor neurons, and the synaptic interactions between the inhibitory and excitatory efferents were analysed by electron microscopy of serial sections. Several novel types of synaptic connections have been observed: (i) inhibitory synaptic input on the axon hillock and initial axon segment; (ii) serial synaptic terminals on the sensory cell body; (iii) simultaneous synaptic contacts of the same inhibitory terminal with sensory dendrites and muscle fibres; (iv) reciprocal synapses between the two types of inhibitory efferents; and (v) inhibitory synapses on the primary inhibitory axon. The possible functional significance of these synapses is discussed in the light of earlier electrophysiological and pharmacological findings.

Immunocytochemical evidence for the GABAergic innervation of the stretch receptor neurons in crayfish

The GABAergic innervation of the stretch receptor neurons of the crayfish Orconectes limosus has been investigated by means of light- and electron microscope immunocytochemistry using an antibody to GABA. Both whole-mount preparations and post-embedding semithin sections revealed a massive GABAergic innervation of both the slowly and the fast adapting receptor neurons. The stretch receptor organ is supplied by one principle GABA-immunoreactive axon, which gives off several branches that innervate the receptor neurons. Cell body, initial axon segment and dendritic region of the sensory neurons are covered by numerous GABA-immunoreactive varicose fibers. Electron microscopy revealed that the GABA-immunoreactive varicosities establish specialized synaptic contacts with the sensory neurons. The functional significance of the occurrence of GABA-immunoreactive varicosities on the different parts of the sensory neurons is discussed. The results support the physiological and pharmacological evidence that GABA is a transmitter substance of the efferent inhibitory neurons which innervate the crayfish stretch receptor neurons.

Nerve terminals and synapses in the cardiac ganglion of the adult lobster Homarus americanus

The cardiac ganglion in the lobster Homarus americanus was examined with a transmission electron microscope. Nerve terminals often existed in large aggregations surrounded by glial and connective tissue elements. Axo-axonic and axo-dendritic synapses were present. Six ultrastructurally different types of nerve terminal, each containing an abundance of vesicles, were distinguished: three formed discrete chemical synapses as indicated by typical release site morphology; three did not. The latter appear to be neurosecretory axon terminals of extrinsic neurons. More than one morphologically distinct type of synaptic vesicle occurred commonly in a given terminal, suggesting the presence of coexisting neurotransmitters and/or neuroregulatory factors. Symmetrical chemical synapses and electrotonic junctions between axons were present.

Structure and localization of synaptic complexes in the cardiac ganglion of a portunid crab

The cardiac ganglion of Portunus sanguinolentus exhibits spontaneous rhythmic activity when isolated. The ganglion contains five large and four small intrinsic neurons and is innervated by three pairs of fibres originating in the thoracic ganglia. We have identified the processes of the large neurons in electron micrographs by injecting these cells with two electron-dense markers, horseradish peroxidase (HRP) and Procion Rubine (PR). In addition we have studied the processes of the four smaller neurons by light microscopy serial reconstructions and by electron microscopy of selected regions. Both markers were found only in neuronal processes and not in glial cells nor in the extracellular space, except close to the soma of the injected cell. We found contacts between the small secondary (collateral) processes of the large cells but not between their somata or their primary processes (axons and dendrites). Two specialized structures present at the contacts between the collateral processes were small membrane close appositions, possibly the site of electrotonic junctions, and chemical synapses. Contacts between processes marked by HRP and those marked by PR were common, as were contacts between processes marked by either HRP or PR and those of the other intrinsic neurons. Adjacent processes stained by PR could contain PR deposits of different densities, but it is unclear whether this finding was due to intercellular diffusion of the dye or to its diffusing at different rates into branches of the same process. Identified processes of all the intrinsic neurons contained the same type of vesicles, which were different from those found in processes of the extrinsic fibres. Chemical synapses were present at contacts between processes of the extrinsic and intrinsic neurons, as well as at contacts between processes of the intrinsic neurons. The axons of three small cells made a series of contacts at which extensive arrays of membrane close appositions, but not chemical synapses, were found. These three axons also formed contacts, either directly or through their collateral branches, with processes of the large cells, at which both membrane close appositions and chemical synapses were present. The axon of the fourth small cell could not be followed in our series.

Intramembranous organization of lobster excitatory neuromuscular synapses

The fine structure of identified neuromuscular synapses of the single excitatory axon to the distal accessory flexor muscle in lobster limbs was examined with freeze-fracture and serial thin-section electron microscopy. The latter technique reveals presynaptic dense bars with synaptic vesicles aligned on either side of these bars and often fused to the membrane, suggesting exocytosis and confirming our previous contention that these bars are active zones of transmitter release. The intramembranous organization of these active zones, as revealed in freeze-etched tissue, is a ridge-like elevation of the P-face of the axolemma with a matching trough on the complementary E-face. The ridge on the P-face has rows of large scattered intramembranous particles along the apex and is often bordered by a series of small, circular depressions which are presumed to represent exocytotic vesicles attached to the presynaptic membrane. Complementing these depressions are a few volcano-like protuberances seen occasionally on the E-face membrane. Because such evidence for transmitter release occurred in both stimulated and non-stimulated preparations, it demonstrates that chemical fixatives employing aldehydes induce transmitter release. The postsynaptic receptor sites of these excitatory synapses are characterized by oval-shaped patches of densely packed particles on the E-face, arranged in a random pattern on the sarcolemma. The complementary P-face view exhibits a regular square array of particle imprints or pits.

Freeze-fracture study of an invertebrate multiple-contact synapse: the fly photoreceptor tetrad

Bow-shaped particle arrays on the P-faces of the photoreceptor terminals R1-R6 in the lamina ganglionaris of the house fly represent the presynaptic sites of chemically mediated multiple-contact synapses (Shaw and Stowe, '82, Saint Marie and Carlson, '82). A particle array consists of two polar patches of regularly arranged particles and a central patch of irregularly arranged ones. Corresponding to these P-face arrays, the receptor E-faces have lattices of pits opposite the polar patches, and pits and some particles at the center. The presynaptic particle array corresponds in its dimensions to the electron-dense bar found in thin sections. The center-to-center spacing of the regularly arranged particles agrees with the spacing of striations found in the bar overlying the two polar elements of the postsynaptic tetrad. The elements in the two medial postsynaptic positions are hyperpolarizing monopolar cells L1 and L2, which show a strip of P-face particles within an otherwise bare postsynaptic membrane enclosed by a ridge, and a bare E-face. Comparison with other invertebrate synapses reveals two types of organization of postsynaptic membranes. IMPs fracture with the postsynaptic P-face in GABAergic and/or inhibitory synapses and with the E-face in glutaminergic and/or excitatory synapses; the fly photoreceptor synapse thus fits in the former category.

Intramembrane organization of synapses in the lobster stretch receptor organ

The intramembrane organization of axodendritic and neuromuscular synapses in the lobster stretch receptor organ was investigated by freeze-fracturing. Based on ultrastructural criteria which are known to be correlated with physiological properties, we identified three types of synapse: the inhibitory axodendritic, the inhibitory neuromuscular, and the excitatory neuromuscular synapse. Although these synapses have some features in common, each has a characteristic arrangement of intramembrane particles in both the presynaptic and postsynaptic membranes. All three have, in their presynaptic membranes, aggregates of P-face particles and associated depressions representing sites of synaptic vesicle exocytosis, features which together define active zones. However, in the inhibitory axodendritic synapse the P-face contains short ridges in this region. These ridges may occur singly or in pairs oriented in V-shaped configurations. The ridges are decorated with particles along their entire length. In the inhibitory neuromuscular synapse, no ridges are present. Clusters of particles are present, but they are scattered randomly over a large expanse of presynaptic membrane. In the excitatory neuromuscular synapse, isolated clusters of particles are associated with the P-face and are occasionally located on circular elevations of the membrane. The postsynaptic membrane also shows structural diversity in the three types of synapse. In the inhibitory axodendritic synapse, there is no apparent specialization. However, in the inhibitory neuromuscular synapse, P-face particles are arranged in double rows which are separated by particle-free strips of membrane. In the excitatory neuromuscular synapse, particles are confined to a narrow band that borders the synaptic cleft. This band is demarcated by a single intermittent strand of particles arranged in the direction of the long axis of the muscle fibre. Therefore, intramembrane specializations of both the presynaptic and postsynaptic membranes are sufficiently distinctive that three different types of synapse can be recognized.

The edge cell, a possible intraspinal mechanoreceptor

In the lateral edge of the "white matter" in the lamprey spinal cord, there is a group of nerve cells referred to as edge cells. The results of a combined physiological, light microscopical, and electron microscopical study suggest that these cells serve as intraspinal mechanoreceptors. Edge cells are depolarized on stretch of the lateral margin of the spinal cord, and they have nestlike ramifications in this region oriented in a rostrocaudal plane. These cells exhibit a close structural similarity with the crayfish stretch receptor.

Inhibitory and excitatory synapses in crayfish stretch receptor organs studied with direct rapid-freezing and freeze-substitution

Crayfish abdominal stretch receptor organs are innervated by inhibitory (GABA) and excitatory (glutamate) synapses. Previous studies with aldehyde fixation showed that synaptic vesicles in the inhibitory synapse are flat and small, whereas those in the excitatory synapse are rounder and larger. We have reexamined these inhibitory and excitatory synapses by using direct rapid-freezing and freeze-substitution in order to preserve synaptic structure closer to its living state. Fine details of synaptic structure appear to be better preserved by this method. Synaptic vesicles in inhibitory as well as excitatory synapses are round, so the conventional flattened shape of vesicles in the inhibitory synapse must depend on some aspect of aldehyde processing. However, the average size of vesicles in the inhibitory synapse is significantly smaller than that of vesicles in the excitatory synapse, so synaptic vesicle size is regarded as having functional significance.

Ultrastructure of the crayfish stretch receptor in relation to its function

The crayfish slow-adapting abdominal stretch receptor was fixed under the relaxed or stretched condition. During this procedure action potentials of the sensory neuron were recorded by a suction electrode. The receptor organ consists of a receptor muscle and a sensory neurons with its dendrites embedded in the connective tissue zone in the receptor muscle. From the cell body of the neuron, several "primary dendrites" arise, branch successively into "dendritic branches", and finally terminate as "dendritic tips," which are cylindrical processes of fairly uniform diameter. In contrast to the primary dendrites and the dendritic branches, the dendritic tips have neither mitochondria nor sheaths and are embedded in the connective tissue zone or apposed to the receptor muscle with a gap of about 15 nm. Microtubules and smooth ER are seen in all parts of the dendrites. When the receptor is stretched and then fixed with 1.6% glutaraldehyde in 0.12 M phosphate buffer (total osmolarity of this solution is isosmotic with the physiological solution), dendritic tips became more parallel to the long axis of the receptor muscle and showed marked deformation consisting of alternate regions of swelling and shrinkage, resulting in a bead-like appearance. When fixed with 1.6% glutaraldehyde in 0.2 M phosphate buffer (total osmolarity of this solution is hyperosmotic), the dendritic tips showed less tendency toward such deformation. These results suggest that the dendritic tip membrane is susceptible to stretch and might be the region where the generator potential is produced.